The present invention relates generally to methods for making cellulosic webs. More particularly, the invention concerns methods for making low-density paper products such as tissue with reduced energy input.
In the manufacture of paper products such as paper towels, napkins, tissue, wipes, and the like, there are generally two different methods of making the base sheets. These methods are commonly referred to as wet-pressing and throughdrying. While the two methods may be the same at the front end and back end of the process, they differ significantly in the manner in which water is removed from the wet web after its initial formation.
More specifically, in the wet-pressing method, the newly-formed wet web is typically transferred onto a papermaking felt and thereafter pressed against the surface of a steam-heated Yankee dryer while it is still supported by the felt. The dewatered web, typically having a consistency of about 40 percent, is then dried while on the hot surface of the Yankee. The web is then creped to soften it and provide stretch to the resulting sheet. A disadvantage of wet pressing is that the pressing step densifies the web, thereby decreasing the bulk and absorbency of the sheet. The subsequent creping step only partially restores these desirable sheet properties.
In the throughdrying method, the newly-formed web is transferred to a relatively porous fabric and non-compressively dried by passing hot air through the web. The resulting web can then be transferred to a Yankee dryer for creping. Because the web is substantially dry when transferred to the Yankee, the density of the web is not significantly increased by the transfer. Also, the density of a throughdried sheet is relatively low by nature because the web is dried while supported on the throughdrying fabric. The disadvantages of the throughdrying method, though, are the operational energy cost and the capital costs associated with the throughdryers.
In the throughdrying process, water is removed by at least two processes: vacuum dewatering and then throughdrying. Vacuum dewatering is initially used to take the sheet from the post-forming consistency of around 10 percent to roughly 20-28 percent, depending on the particular furnish, speed and local energy costs. It is well known that the cost of water removal is relatively low at low consistencies, but increases exponentially as more water is removed. Hence, vacuum dewatering is generally used until the cost of additional water removal becomes higher than that of the succeeding throughdrying stage.
In the throughdrying stage, the energy cost again varies depending on the process and furnish specifics, but in all cases requires a minimum of 1000 BTU/pound of water removed because this is the latent heat of vaporization of water. In practice, generally about 1500 BTU are required per pound of water removed, with the additional BTU's related to the sensible heat needed to bring the water to the boiling point and energy losses in the system. Despite the relatively high energy input required for throughdrying, however, this process has become the process of choice for soft, bulky tissue because of the resulting product quality. For a new tissue machine producing premium quality tissue, it is often profitable to spend the additional capital and energy cost to make the desired product.
But, since the vast majority of existing tissue machines utilize the older wet-pressing method, it is of particular importance that manufacturers find ways to modify existing wet-pressed machines to produce the consumer-preferred low-density products without expensive modifications to the existing machines. Of course, it is possible to re-build wet-pressed machines to throughdried configurations, but this is usually prohibitively expensive. Many complicated and expensive changes are necessary to accommodate the throughdryers and associated equipment. Accordingly, there has been great interest in finding ways to modify existing wet-pressed machines without significantly altering the machine design.
One simple approach to modifying a wet-pressed machine to produce softer, bulkier tissue is described in U.S. Pat. No. 5,230,776 issued Jul. 27, 1993 to Andersson et al. The patent discloses replacing the felt with a perforated belt of wire type and sandwiching the web between the forming wire and this perforated belt up to the press roll. The patent also appears to disclose additional dewatering means, such as a steam blowing tube, a blowing nozzle, and/or a separate press felt, that may be placed within the range of the sandwich structure in order to further increase the dry solids content before the Yankee cylinder. These extra drying devices are said to permit the machine to run at speeds at least substantially equivalent to the speed of throughdrying machines.
It is important to reduce the moisture content of the web coming onto the Yankee dryer in order to maintain machine speed and to prevent blistering or lack of adhesion of the web. Referring to U.S. Pat. No. 5,230,776, the use of a separate press felt, however, tends to densify the web in the same manner as a conventional wet-pressing machine. The densification resulting from a separate press felt would thus negatively impact the bulk and absorbency of the web.
Further, jets of air for dewatering the web are not per se effective in terms of water removal or energy efficiency. Blowing air on the sheet for drying is well known in the art and used in the hoods of Yankee dryers for convective drying. In a Yankee hood, however, the vast majority of the air from the jets does not penetrate the web. Thus, if not heated to high temperatures, most of the air would be wasted and not effectively used to remove water. In Yankee dryer hoods, the air is heated to as high as 900 degrees Fahrenheit and high residence times are allowed in order to effectuate drying.
Thus, what is lacking and needed in the art is a method of making low-density tissue on a wet-pressed machine at conventional wet-pressed speeds, and in particular, a method that produces consumer-preferred low-density products with reduced energy input.